This invention relates to a system that allows emergency vehicles to remotely control traffic signals, and more specifically, a detector for use in such a system, wherein the detector receives pulses of light from an approaching emergency vehicle and transmits a signal representative of the distance of the approaching vehicle to a phase selector, which can issue a preemption request to a traffic signal controller.
Traffic signals have long been used to regulate the flow of traffic at intersections. Generally, traffic signals have relied on timers or vehicle sensors to determine when to change traffic signal lights, thereby signaling alternating directions of traffic to stop, and others to proceed.
Emergency vehicles, such as police cars, fire trucks and ambulances, generally have the right to cross an intersection against a traffic signal. Emergency vehicles have typically depended on horns, sirens and flashing lights to alert other drivers approaching the intersection that an emergency vehicle intends to cross the intersection. However, due to hearing impairment, air conditioning, audio systems and other distractions, often the driver of a vehicle approaching an intersection will not be aware of a warning being emitted by an approaching emergency vehicle. This can create a dangerous situation when an emergency vehicle seeks to cross an intersection against a traffic signal and the driver of another vehicle approaching the intersection is not aware of the warning being emitted by the emergency vehicle.
This problem was first successfully addressed in U.S. Pat. No. 3,550,078 (Long), which is assigned to the same assignee as the present application. The Long patent discloses an emergency vehicle with a stroboscopic light, a plurality of photocells mounted along an intersection with each photocell looking down an approach to the intersection, a plurality of amplifiers which produce a signal representative of the distance of the approaching emergency vehicle, and a phase selector which processes the signal from the amplifiers and can issue a request to a traffic signal controller to preempt a normal traffic signal sequence to give priority to the approaching emergency vehicle.
The Long patent discloses that as an emergency vehicle approaches an intersection, it emits a series of light pulses at a predetermined rate, such as 10 pulses per second, and with each pulse having a duration of several microseconds. A photocell, which is part of a detector channel, receives the light pulses emitted by the approaching emergency vehicle. An output of the detector channel is processed by the phase selector, which then issues a request to a traffic signal controller to change to green the traffic signal light that controls the emergency vehicle's approach to the intersection.
In the Long patent, each detector channel is comprised of two photocells in parallel with an inductor. The photocells also act as capacitors, so that the photocells and the inductor form an LC resonant circuit. The resonant circuit is tuned to oscillate at a predetermined frequency, such as 6 KHz. The capacitance of the photocells and the inductance of the inductor determine the frequency of oscillation.
The inductor also acts as a DC short. Without the inductor, a constant or slowly changing light source, such as the sun or an approaching car headlight, would saturate the photocells and render them ineffective. Therefore, the inductor also acts to make the resonant circuit respond only to quickly changing inputs.
When a photocell is presented with a pulse of light, the resonant circuit produces a decaying sinusoid signal. The signal is amplified and sent to the phase selector. By measuring the magnitude of the decaying sinusoid signal, the phase selector can determine the distance of the approaching emergency vehicle.
Because the system taught by Long is dependent upon the capacitance of the photocells and the inductance of the inductor to produce the predetermined oscillation frequency, each detector channel must always have two photocells. In a typical intersection, there are four approaches. For example, one street may approach an intersection from the east and west and another may approach the intersection from the north and south. In one embodiment, the two photocells in a detector channel can be aimed in opposite directions, for example, one aimed north and the other aimed south. Another detector channel is used for the other street, with one photocell aimed east and the other aimed west. If an emergency vehicle approaches, say from the south, the photocell that is pointed south will activate the north-south detector channel. The detector channel output signal will be processed by the phase selector which will then issue a request to the traffic signal controller to change the traffic signal lights to green in the north and south direction and to red in the east and west direction. The traffic signal lights are now set such that the emergency vehicle can proceed through the intersection and cross traffic will be required to stop.
In another embodiment, a typical four approach intersection will use four detector channels, with each detector channel having its two photocells pointed in approximately the same direction. In this embodiment, when an approaching emergency vehicle is detected, the traffic signal lights on three of the approaches will change to red. The traffic signal lights controlling the emergency vehicle's approach will change to green.
This embodiment requires four more photocells than are physically needed to detect all approaches because the detector circuit disclosed by Long must have two photocells per detector channel to create the capacitance required for the resonant circuit to oscillate at the predetermined frequency. Long does not disclose a circuit or method that can have a variable number of photocells per detector channel.
The resonant circuit disclosed by Long creates another problem; the inductor acts as an antenna and induces noise into the circuit. The detector circuit requires extensive shielding to minimize noise.
U.S. Pat. No. 4,704,610 (Smith et al) also discloses an emergency vehicle traffic control system. The Smith et al patent discloses an emergency vehicle that transmits infrared energy to a receiver mounted near an intersection. The infrared energy transmitted by the emergency vehicle preferably has a wavelength centered at approximately 0.950 micrometers and is modulated with a 40 KHz carrier.
The infrared receiver of Smith et al is comprised of a photovoltaic detector in parallel with a tunable inductor. The tunable inductor is adjusted to allow only signals modulated with a 40 KHz carrier to be detected by the amplifier/demodulator circuit. The tuned photovoltaic detector/inductor circuit effectively eliminates DC signals from background solar radiation.
The detector circuit disclosed by Smith et al suffers from the same problems as the detector circuit disclosed by Long; it is impossible to change the number of photocells per detector channel without having to retune a resonant circuit to maintain a predetermined frequency. Also, the inductor disclosed by Smith et al, like the inductor disclosed by Long, is likely to act as an antenna and therefore introduce radio frequency noise into the detector circuit.